1. Field of the Invention
The invention relates to a device for measuring biological components, especially live cells, that has at least one field effect transistor, which has on a substrate a source, a drain, and a channel area that connects said source and drain. On top of the channel area a gate electrode is arranged, which is electrically insulated from the channel area by a thin insulation layer.
2. Description of Related Art
Such a device is disclosed in DE 196 23 517 C1. It has a field effect transistor in which the gate electrode is electrically connected by a circuit path to an open contact pad surrounded by an electrical insulator. The dimensions of the open contact pad are constructed such that it can contact a live biological cell contained in a nutrient solution. Such a device allows the extracellular measurement of the action potential of a cell that is mounted onto the contact pad, especially a nerve or muscle cell. The substrate of the field effect transistor consists of silicon, in which a tub-like semiconductor layer of a first charge carrier has been set. In this semiconductor layer, endowed drain and source regions are arranged, in between which a channel area is formed. On top of the channel area is a thin insulation layer and on top of it the gate electrode. The gate electrode consists of poly-silicon and covers the complete channel area as well as the neighboring edges of the drain and the source. The gate electrode forms an isoelectric region that distributes an electrical potential bordering it over the complete channel length stretching from the drain to the source, such that the potential reaches also the places where the channel area shows its highest sensitivity even if the field effect transistor is saturated when an asymmetrical, one-sided distribution of free charge carriers in the channel area occurs along the channel length. The disadvantage of the device, however, is that its measurement sensitivity is strongly reduced if the contact pad that is connected to the gate is only partially covered by the cell such that the nutrient solution, which contains the cell, contacts other regions of the contact pad. The decrease in measurement sensitivity occurs mainly because the voltage, which is essentially capacitatively interlinked by the biological cell into the contact pad and thus also the gate electrode, of the initial voltage corresponds to a replacement voltage source with high source resistance. Due to the ions contained in it, the nutrient solution is relatively low-ohmic in comparison to the source resistance. Thus, the measured signal next to the gate is correspondingly reduced when the replacement voltage source is burdened with the electrical resistance of the nutrient solution. The cell signal to be measured is then essentially short-circuited due to the nutrient solution lying over a reference potential, that is, the majority of the voltage does not occur on the gate but rather drops due to the source resistance of the replacement voltage source. It is also unfortunate that the arrangement consisting of the gate electrode, the conducting path, and the contact pad has a relatively large electrical capacitance for the nutrient solution, which in addition weakens the measurement signal.
Therefore, the objective is to design a device as mentioned above, wherein the danger is reduced that the measurement signal results through a contact of the gate electrode with a nutrient solution containing the biological component(s) to be measured.